Our studies showed that focal brain ischemia produced by three hour occlusion of the middle cerebral artery in cats produced a rise in the cerebral electrical impedance of the affected grey matter. The CEI returned approximately to pre-ischemic levels when the occlusion was released. In most of these animals a second rise in CEI was observed to occur later which appeared to be related to an increase in intracranial pressure. To test this hypothesis, brain compression was produced by epidural balloon inflation. When the epidural pressure was increased, the CEI increased as much as 216%. The regional blood flow (rCFB) was lowered but not necessarily to ischemic levels. This finding indicated that brain compression produced by edema can itself produce a reduction in extracellular space without always lowering rCBF to critical ischemic levels.